1. Field of the Invention
The present invention provides a method for conveying teletype information over a wireless communication system and more particularly to a method in which the teletype information is encoded so that it can be reliably conveyed over the wireless telecommunication system.
2. Description of the Related Art
Teletype terminals are part of telecommunication technology developed more than 30 years ago. The teletype terminals are equipment that were designed to transmit and receive teletype signals where such teletype signals contain information arranged in accordance with a well known signaling protocol such as Baudot signaling. A signaling protocol is a set of rules followed by a provider of a communication system that defines how information is to be encoded, transmitted, received and decoded by equipment of the communication system. The Baudot signaling protocol has remained virtually unchanged for the past 30 years. Baudot signaling is an analog signaling protocol in which analog signals are used to digitally encode information that are conveyed over a telephony system such as the Public Switched Telephone Network (PSTN).
There are still many active teletype terminals in the United States and throughout the world. Many of these active terminals still transmit and receive information in accordance with the 45.45 baud Baudot analog signaling protocol or other similar protocols. In the 45.45 baud Baudot signaling protocol, each character is coded as an 8 bit block of information comprising 1 start bit, 5 information bits and 2 stop bits where each bit is 22 milliseconds in length. Thus, 45.45 baud (1/22 msec.) represents the speed at which the information is conveyed and can be generally referred to as the information rate. The actual information is represented by the 5 information bits. The start bit, which is usually defined as a xe2x80x9c0xe2x80x9d bit and the stop bits, which are usually defined as xe2x80x9c1xe2x80x9d bits, indicate the beginning and end of each 8 bit block of information respectively. For many signaling protocols, including the Baudot signaling protocol, the 8 bit block of information usually represents an alphanumeric character. An 8 bit block of information is commonly referred to as an 8 bit word. The start and stop bits are typically referred to as synchronization bits and are part of signaling information. Signaling information is information used by the system that enable the system to properly transmit, receive and process the user information. The Baudot signaling protocol being an analog signal protocol, each bit is represented by a tone (i.e., a single frequency sinusoid). Thus, a xe2x80x9c0xe2x80x9d bit is represented by an 1800 Hz tone and a xe2x80x9c1xe2x80x9d bit is represented by a 1400 Hz tone.
With the advent of wireless telecommunication systems, owners of teletype terminals have attempted to transmit and receive teletype signals over such systems. Moreover, the Federal Communications Commission (FCC) has adopted rules requiring providers of wireless telecommunication systems to ensure that teletype users can access Emergency 911 services over wireless telecommunication systems. The Emergency 911 services are the well known types of services typically provided via the PSTN (or other communication network) in which a subscriber of the PSTN simply dials 911 to obtain emergency assistance from local authorities (e.g., police dept., fire dept., hospital/ambulance services). However, system providers have discovered that the teletype terminals have performed poorly over wireless telecommunication systems. One indication of poor performance is the ratio of the number of erroneous characters received to the total number of characters transmitted; this ratio is known as the Character Error Rate (CER). Several studies and trials conducted by various wireless equipment manufacturers confirm the poor performance (e.g., unacceptably high CER) of teletype communications over wireless telecommunication systems. The following are some of the more recent studies: E911 TDD Compatibility Testing With IS-136, Jan. 22, 1998, Ericsson Corp.; E911 TDD Compatibility to GSM, 1998, Ericsson Corp.; TTY Study using Vocoder direct link Gallaudet University Technology Assessment Program; TTY/TDD Compatibility Measurements Preliminary Resultsxe2x80x9d, May 1998, Nokia Corp.; TDD Communications over CDMA September 1997, Qualcomm Inc.; xe2x80x9cInitial TTY Reportxe2x80x9d May 1998, Sprint PCS.
It is widely thought that the poor performance of teletype communications over wireless telecommunication systems is due to the use of vocoders in such systems. Vocoders are speech coding systems designed to represent human voice as mathematical models comprising various parameters. Also, vocoders comprise speech coding systems that simply digitize analog voice signals into Pulse Coded Modulation (PCM) samples (i.e., bit streams) without any mathematical parameter modeling. The parameters are transmitted and received over wireless telecommunication channels. At the transmit end, a vocoder produces a set of parameters for every basic timing interval of the wireless telecommunication system. The basic timing interval is part of a format being followed by a wireless telecommunication system. The format is a particular method of arranging and structuring information to be conveyed over a wireless telecommunication system. The basic timing interval is a period of time during which a block of information representing, for example, voice, data, video and other communication signals, is conveyed. For certain wireless telecommunication systems such as Code Division Multiple Access (CDMA) wireless telecommunication systems, the basic timing interval is commonly referred to as a frame. Each frame contains a plurality of digital bits representing digitally encoded information. At the receive end, another vocoder uses the received parameters to reproduce human voice. In current CDMA wireless telecommunication systems the frame is 20 milliseconds long and some of the vocoders used are the QUALCOMM Code Excited Linear Predictive Coder (QCELP 13 k or 8 k) and the Enhanced Variable Rate Coder (EVRC).
Signals that do not originate from human speech such as tones are sounds that can adversely impact the quality and accuracy of the mathematical model parameters produced by the vocoders, thus, the ability of a vocoder at the receive end to accurately reproduce such signals would also be adversely affected. Many have reasoned that since teletype signals (i.e., tones) are not sounds typically produced by human voice, some vocoders have difficulty in reproducing such signals resulting in the poor performance of teletype signals when conveyed over wireless telecommunication systems. However, tests and performance studies have shown that the poor performance of teletype signals conveyed over wireless telecommunication systems is mainly due to the quality of the communication channels of these systems and not to the use of vocoders. The quality of these communication channels may be such that the conveyed teletype signals are damaged to the extent that an unacceptably high information error rate (i.e., high CER) results.
Referring to FIG. 1, there is shown a test setup designed to evaluate the performance of teletype signals over a communication channel of a CDMA system using QCELP 13k vocoders. At the transmit end teletype signals from a teletype terminal (not shown) are fed to vocoder which produces the mathematical model parameters. The parameters produced by vocoder 100 are transmitted as frames over communication channel 102. In this setup, communication channel 102 is designed to be ideal in the sense that the transmitted frames experience no adverse effects. Adverse effects are any existing conditions in the wireless telecommunication system which cause errors to occur in the information being conveyed through the system. A non-zero information error rate, which is usually defined as the ratio of erroneous information to total information, directly results from the adverse effects.
In an actual CDMA wireless telecommunication system, the frames would be transmitted as radio frequency (RF) signals over the air and/or other communication channel media. Depending on the quality of the communication channels being used, a certain percentage of the transmitted frames would be adversely affected and thus such frames (i.e., xe2x80x9cbad framesxe2x80x9d) would contain erroneous bits. Usually, a Frame Error Rate (FER) is associated with a wireless telecommunication system where the FER is typically defined as the ratio of bad frames to the total number of frames transmitted through communication channels of the wireless telecommunication system. Communication channel 102 has a 0% FER. For other wireless communication systems, a corresponding information error rate known as the Bit Error Rate (BER) is an indicator of the quality of the communication channels for those systems.
Vocoder 104 receives the transmitted frames and reproduces the teletype signals whose CER is then measured. It is found that at 0% FER the CER of the teletype signals is also 0% for the test setup of FIG. 1. In a test of an actual CDMA system using a QCELP 13k vocoder it was found that an FER barely under 1% yields a CER of about 7%. The same test conducted for a Time Division Multiple Access (TDMA) wireless telecommunication system, which uses a different type of vocoder known as an Algebraic Code Excited Linear Predictive (ACELP) vocoder, yielded the same results. Similar tests for other types of wireless systems such as Global System for Mobile Communication (GSM) yielded similar results. Moreover, under acceptable communication channel conditions (nearly 0% FER or 0% BER), different types of wireless telecommunication systems (e.g., CDMA, TDMA, GSM, PCS) yielded about a 1% CER for teletype signals. However, for a 1% FER or higher, the CER increased dramatically.
A quantitative analysis of teletype signals transmitted through a less than ideal communication channel of a CDMA wireless telecommunication system further confirms that the dominant cause of the poor performance of the teletype signals is due to the relative quality (manifested by a relatively high PER, or high information error rate) of the communication channel. In particular, for a CDMA wireless telecommunication system, 300 teletype characters were transmitted through a channel whose FER is 1%. The teletype characters complied with the 45.45 baud Baudot signaling protocol. Each frame of the CDMA system is 20 milliseconds long. Therefore the number of frames in 300 characters is 2640 frames (i.e., 300 char.xc3x978 bits/char.xc3x9722 msec./bitxc3x97(1 frame/20 msec)). A 1% FER is therefore equivalent to 26.4 bad frames for 300 transmitted characters. In a worst case scenario each bad frame produces one bit error in a different character leading to 26.4 character errors. The CER is thus (26.4/300)xc3x97100=8.8%. The relationship between the FER and the CER is defined by the formula CER=8.8 FER. Under a typical CDMA system, the average FER is 2% which could result in a CER of up to 17.6% for teletype communications.
Based on the above discussion it is evident that the dominant factor that affects teletype communications over wireless telecommunication systems is the information error rate (e.g., FER, BER) of the system. In order to achieve reliable teletype communications over a wireless telecommunication system, the effect of the system""s information error rate on the teletype signals must somehow be attenuated as much as possible.
Therefore, there exists a need to convey (i.e., transmit and receive) teletype information over wireless telecommunication systems in such a manner that any adverse effects (manifested as information errors) on the teletype information due to the quality of the communication channels of the system is substantially attenuated.
The present invention provides a method for conveying teletype information over a wireless telecommunication system in a reliable manner. Equipment of the wireless telecommunication system receive teletype signals carrying teletype information. The teletype information is encoded as a repetitive signal and is transmitted over the wireless telecommunication system. The repetitive signal is a signal encoded with the teletype information and said coded signal is replicated at least once and concatenated to the original coded signal. The repetitive signal is thus a string of replicated coded signals. The teletype information is thus advantageously repeated within the repetitive signal such that when the repetitive signal is received after having been transmitted over the wireless telecommunication system, the teletype information can still be retrieved from part of the repetitive signal determined as not having been adversely affected by the wireless telecommunication system.
Therefore, equipment of the wireless telecommunication system, such as a base station, are configured to receive teletype signals, repetitive signals carrying teletype information in addition to voice signals and other non-teletype signals. The equipment applies the method of the present invention as follows: First, a signal is received. The received signal is then encoded as a repetitive signal and is transmitted over the wireless telecommunication system when it has been determined that the received signal is a teletype signal. Teletype information is retrieved from portions of the received signal determined as not having been adversely affected by the wireless telecommunication system when it has been determined that the received signal is a repetitive signal carrying teletype information.